Lack of glycogenin causes glycogen accumulation andmuscle function impairment

Glycogenin is considered essential for glycogen synthesis as it acts as a primer for the initiation of the polysaccharide. In this study, we challenge this notion and demonstrate that glycogen can be synthesized in vivo in the absence of glycogenin. Glycogenin-deficient mice (Gyg KO) accumulate high amounts of the polysaccharide in skeletal and cardiac striated muscles. This glycogen shows no covalently bound protein, thereby indicating that no protein primer is essential for glycogen synthesis. Gyg KO mice show lower resting energy expenditure and lesser resistance when subjected to endurance exercise than control animals, which can be attributed to a switch of oxidative myofibers toward glycolytic metabolism. This switch is caused by the over-accumulation of glycogen, since mice overexpressing glycogen synthase specifically in skeletal muscle show a similar metabolic alteration. These results may explain the muscular defects of GSD XV patients, who show high glycogen accumulation in striated muscles.